In most power systems the voltage level will tend to vary due to several factors such as load, line capacitance or line resistance. This variation is disagreeable to the customer since it could result in poor performance or even equipment damage.
A step-voltage regulating transformer is a device which is often used to maintain the voltage of a system relatively constant. The voltage is maintained relatively constant by an apparatus which: (1) detects changes in the system voltage; and (2) automatically adjusts system voltage without interrupting service. An early step voltage regulating transformer is disclosed by Sealey in U.S. Pat. No. 2,713,142.
There are two techniques used by the regulator industry for sensing changes in system voltage. One technique, widely used by the industry, uses a tapped auto-transformer circuit. Normally, it consists of a tapped series winding on an auto-transformer that facilitates plus or minus 10 percent regulation; a shunt winding across the regulator input terminals; a potential winding closely coupled to the shunt winding; and a current transformer primary winding in the load line. A reversing switch that is always in either a "raise" or a "lower" position, depending on whether the regulator has been boosting or bucking the source voltage, is disposed across the ends of the series winding. With the reversing switch in the raise position, the series winding becomes additive with respect to the shunt winding as the number of turns placed in series with the load increases. The amount of voltage boost therefore increases. When the reversing switch is moved to the lower position, the series windings, therefore, become subtractive with respect to the shunt winding and the amount of voltage buck depends upon the number of turns placed in series with the load. An automatic circuit controls the amount of voltage boost or buck.
The potential winding is coupled to the shunt winding with a turns ratio depending upon the regulator voltage rating. Thus, the sensing signal (i.e. the voltage produced across the potential winding) is related to the input voltage or source voltage by the ratio of turns between the potential winding and the shunt winding. The output of the potential winding is an accurate reflection of changes in source voltage. Since the potential winding is wound as an integral part of the main transformer, this is a relatively inexpensive way of providing a sensing signal for the automatic adjusting circuit of the voltage regulator.
Another technique for sensing changes in system voltage employs a separate potential transformer. This transformer simply steps down the output voltage, which is subject to variations, to a usable level. Since a separate transformer is used, it must be designed for the full line voltage being regulated by the transformer. This is a much more expensive technique, relative to the method previously described. U.S. Pat. No. 3,054,943 to Kettler et al. and U.S. Pat. No. 3,184,677 to Jacobsen disclose two control circuits adopted to be used with such a potential transformer.
In both techniques, the automatic voltage regulator, or the automatic device which changes the tap settings of the series winding, is not responsive to voltage changes due to current flowing both into and out of the input terminals to the transformer. Those skilled in the art know that in the case of multiple feed systems or feed systems employing alternate power sources it is possible for reverse power flow to occur. Unless the automatic voltage regulating portion of the transformer is arranged to be responsive to current flowing in either direction, instability is likely to occur. The traditional solution to this problem has been to employ a separate potential transformer across the output terminals of the regulating transformer and to use a potential winding electromagnetically coupled to the shunt windings to sense the changes in the voltage supply to the input terminals. Power flow in one direction was then sensed by the potential transformer while power flow in the opposite direction was sensed by the potential winding.
What is needed is a simple, relatively inexpensive circuit which can be readily adapted to existing distribution transformer voltage regulating designs and which can be used to control the automatic regulating circuitry of the transformer whether or not current is flowing into or out of the input terminals of the transformer.
Preferably, existing windings and terminals would be used to the maximum extent possible. Such a circuit would reduce the size, weight and material requirements necessary to produce a transformer, the overall cost of the transformer, the cost of maintaining and using the transformer and would simplify the overall design of the automatic control circuitry of the transformer.